1. Field of the Invention
This invention relates to photodetectors. Still more particularly, this invention relates to photoconductors which are composed of two different types of photodiodes.
2. Description of the Prior Art
The class of photodetectors involved herein finds its main usage in optical tracking system applications. The most common photodetector utilized for such applications is a multi-segmented silicon reach through avalanche photodiode (RAPD). A typical such reach through avalanche photodiode will have a three millimeter diameter and be divided into four pie-shaped segments. The photodetector itself is but one component in a larger tracking system comprising a pulsed optical source which impinges upon and is reflected from a target, the reflected echo signal, appropriate receiver optics which form a focused or defocused image on the photodetector, and appropriate control circuits actuated by the signals from the photodetector. The reflected optical pulses impinge upon the photodetector and provide ranging and angular information to the control circuits since any deviation of the target image from bore sight will result in unequal illumination of the four segments of the photodetector. A stronger or weaker signal from one or more quadrants will then energize appropriate control circuits in the device whose orientation is controlled by the tracking system which will then cause the device to re-attain a bore sight tracking attitude in reference to the target.
The diameter of the conventional reach-through avalanche photodiode photodetector device represents an unhappy compromise between two antagonistic constraints. The first is that the diameter of the photodetector must be large enough to accommodate a large target image at close range. The opposing consideration is that, with a conventional RAPD device of a typical three millimeter diameter, large amounts of noise, due to the high sensitivity of the RAPD device, will be introduced into the tracking system; and, at extremely close ranges, the large reflected target image on the device will cause overload of the preamplifier circuits leading into the control circuits for the device.
The conventional large area RAPD device has several disadvantages. In a typical application designed for a three degree field of view, the background noise generated by the large area device will substantially reduce the usable range of the guidance system. Actually, a one degree field of view would be adequate for operations down to ranges of about three kilometers and would produce a target image on the photodetector under 0.5 millimeters in diameter. A one millimeter diameter RAPD device would be adequate at this range, and background noise would be reduced by a factor proportional to the ratio of the different field of views, about three. As mentioned above, another problem arises when the target comes into close range from the seeker. In this case, the reflected signal may be sufficiently large to saturate the RAPD device and/or the associated preamplifier. At these close ranges a reduction in avalanche gain or no avalanche gain is desirable. Another disadvantage for conventional large area RAPD devices is caused by processing constraints. It is quite common that materials and processing tolerance limitations will lead to nonuniform avalanche gain over the area and from segment to segment. The result is that matching of segments, particularly over a wide temperature range, is very difficult.
There are no present photodetector devices that operate satisfactorily over the necessary range of target ranges.